Acoustic and shock waves are traveling pressure fluctuations which cause local compression of the material through which they move. Acoustic waves cause disturbances whose gradients, or rates of displacement are small--on the scale of the displacement itself. Acoustic waves travel at a speed determined by and characteristic of a given medium; thus, one must speak of the speed of sound, or acoustic speed in that medium. An acoustic wave regardless of its frequency (pitch) or amplitude (loudness), will always travel at the same speed in a given substance.
Shock waves are distinguished from acoustic waves in two key respects. First, shock waves travel faster than the speed of sound in any medium. Secondly, local displacements of atoms or molecules comprising a medium caused by shock waves are much larger than for acoustic waves. Together, these two factors produce gradients or rates of their displacement much larger than the local fluctuations themselves.
Energy is required to produce pressure waves. This is related to the equation that states that energy equals force multiplied by the displacement caused by the force. Once the driving source ceases to produce pressure disturbances, the waves decay. Attenuation involves acceleration of the natural damping process, which therefore means removing energy from pressure waves.
All matter through which pressure waves travel naturally attenuates these waves by virtue of their inherent mass. Materials possess different acoustic attenuating properties, strongly affected by density and by the presence or absence of phase boundaries and structural discontinuities. Porous solid materials, thus, are better attenuators of sound waves than perfect crystalline solids. Gases are inherently poor pressure wave attenuators.
All types of pressure waves can be reflected and diffracted by liquid and gas media. They can also be deflected or, more generally, scattered and dispersed by phase boundaries, such as liquid droplets or solid particulates suspended in air. These deflections serve to increase the distance which the wave travels. Scattering and dispersion thus produce more attenuation because they cause the transmitting pressure waves to displace more mass by virtue of the longer path. Such deflections also reduce, or may altogether eliminate the pressure waves originally traveling in a specific direction.